Preparation of nitro compounds by vapor phase nitration of aldehydes

ABSTRACT

A process for selectively forming nitro compounds by contacting, at elevated temperature and pressure and in a homogeneous gas phase, an aldehyde having from two to ten carbon atoms with nitrogen dioxide alone or in the presence of oxygen and/or water.

This application is a continuation-in-part application of copending U.S.application Ser. No. 510,860, filed July 5, 1983, now abandoned.

BACKGROUND OF THE INVENTION

The present invention is directed to a process of forming anitroparaffin and nitroaromatic compounds by gaseous phase reaction ofan aldehyde with nitrogen dioxide. The present process provides a methodto form pre-selected nitro compounds based on the particular aldehydefeed. The process further alleviates certain processing steps requiredin the previously used nitration of hydrocarbon feed such as ethane,propane and the like.

Processes to form nitroparaffins by gaseous phase nitration are known.U.S. Pat. Nos. 3,780,115 and 3,869,253 teach that nitration of saturatedhydrocarbons higher than methane can be accomplished by contacting thehydrocarbon feed with nitrogen dioxide in the presence of oxygen, suchas in the form of air. The reactant gases are preheated and thenintroduced into the reaction zone where the gaseous phase nitration iscarried out at elevated pressure and at elevated temperature. Thegaseous effluent emitted from the nitration reaction zone is rapidlyquenched. The quenched mixture then enters a separator where the gaseousmaterials in the form of unreacted hydrocarbon, nitric oxide, carbonmonoxide and carbon dioxide are removed for subsequent purification andrecycling and the remaining phase liquid materials are separated bydecantation and the nitroparaffins are recovered by distillation. Thisnitration process yields a mixture of products having a predominance ofnitropropane and nitroethane.

French Publication 78/32,118 discloses that the nitroparaffins productmixture can be made to have an increased yield of nitromethane, the mostcommercially desired product, by utilizing ethane as the hydrocarbonfeed in the homogeneous gas phase nitration. The nitration process canbe further enhanced by recycling into the hydrocarbon feed some of thenitropropane product and/or by conducting the nitration in the presenceof an inert gas such as nitrogen, hydrogen or argon.

U.S. Pat. No. 4,260,838, similar to the above French reference, teachesthat the gas phase nitration process of U.S. Pat. Nos. 3,780,115 and3,869,253 can be improved by altering the feed stock to obtain suitablepercentages of different nitroparaffins as suits the needs of themarketplace. This patent teaches that the feed stock be made up of amixture containing propane, preferably recycled nitroparaffin andpossibly inert gas and/or another alkane. The nitrating agent can beeither nitrogen dioxide or nitric acid.

Each of the conventional processes, such as those in the abovereferenced patents, relies on the use of a hydrocarbon feed whichprovides a nitroparaffin product mixture. These processes have thefurther defect of providing low yield of nitroparaffin mixture and lowselectivity of the most commercially desired compound, nitromethane.Finally, because of the low yield, processes which are based on thegaseous phase nitration of saturated hydrocarbons produce a large volumeof gaseous reaction effluents composed predominantly of unreactedhydrocarbon. In order to enhance these prior art processes, theunreacted hydrocarbons must be separated and recovered from theremaining gases, such as by cryogenic means, and then recycled as partof the process feed. Such separation and recovery required additionalequipment and adds to the processing costs of the known processes.

A method to selectively form particular nitroalkanes or nitroaromaticsfrom easily available and processable feed is highly desired. It isparticularly desired to have a process to selectively form nitromethane,a very industrially useful product.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a process by which aselective nitroparaffin can be formed or that a selective nitro compoundis the predominant compound of the resultant products.

Another object of the present invention is to provide a process by whichthe various unreacted feed materials are readily separated andrecyclable.

Another object of the present invention is to provide a process by whichone can selectively form nitromethane from readily available andprocessable materials.

The process of the present invention is capable of selectively formingparticular nitrohydrocarbon compounds by contacting at elevatedtemperature and pressure in a homogeneous gas phase a C₂ to C₁₀ aldehydewith nitrogen dioxide, preferably in the presence of oxygen and/orwater.

DETAILED DESCRIPTION OF INVENTION

A process for selectively forming particular aromatic or aliphatic nitrocompounds comprises contacting under homogeneous gas phase reactionconditions an aldehyde with nitrogen dioxide, preferably in the presenceof oxygen and/or water.

The prpcess of homogeneous nitration is generally performed by initiallypreheating the reactants before they are carried into the reaction zone.The preheating conditions are preferably substantially the sametemperature and pressure as the reaction conditions, as fully describedbelow.

The reactant feed of the present process can be selected from aliphaticor aliphatic-aromatic aldehyde compounds or mixture thereof. The term"aldehyde" as used in the present disclosure and in the claims appendedhereto refers to compounds having at least one carboxyl oxygen atomcovalently bonded to a carbon atom, having from two to ten, preferablytwo to five carbon atoms for aliphatic compounds or seven to ten,preferably seven to eight carbon atoms for aromatic ring containingcompounds, and having the carbon atom which is bonded to the oxygen befurther bonded only to hydrogen and carbon atoms. The preferredaldehydes are aliphatic aldehydes. In view of nitromethane being themost commercially important product, the most preferred compound isacetaldehyde.

The particular structure of the aldehyde used as the feed in the subjectprocess will be the determinative element as to what nitro compound isto be formed or what predominant nitro compound is to be formed from amixture. For example, when the aldehyde is represented by ##STR1## inwhich R represents an alkyl, alkaryl or aryl, RNO₂ will be the sole ordominant product formed. The most preferred aldehyde is represented whenR is methyl.

Examples of aldehydes which are useful as a feed in the subject processare acetaldehyde, propionaldehyde, butyraldehyde and the like. Thespecific compound will be dictated by the desired nitro compound and theeconomics and availability of the aldehyde feed.

The aldehyde compounds useful in the present process preferably do notcontain non-hydrocarbon groups except for the carboxyl oxygen, asdescribed above. However, the compounds may contain non-hydrocarbongroups which will not inhibit the subject process, such as nitriles andthe like. The supply of aldehydes may also contain small amounts ofother compounds, such as lower or higher homologs of the aldehydedescribed above and/or other oxygen atom containing compounds withoutinterferring with the presently obtained unexpected result.

The above described aldehydes are contacted in the reaction zone withnitrogen dioxide. The nitrogen dioxide, per se, or presursors (such asN₂ O₄ or HNO₃) capable of forming and providing NO₂ under reaction zoneconditions are feed materials readily obtainable commercially. The terms"nitrogen dioxide","nitrogen peroxide" or "NO₂ " as used in thisdisclosure and in the appended claims shall each refer to the compoundNO₂ or its precursors except when used to describe the reactant in thereaction zone wherein the terms shall each refer to the compound NO₂,per se.

It is preferred that the feed also includes oxygen, usually in the formof air. The oxygen as well as the nitrogen dioxide can be at leastpartially obtained from recycled unreacted materials which have beenseparated and purified by conventional methods from the reaction productas more fully described below.

The feed may further contain inert gas such as nitrogen, carbonmonoxide, carbon dioxide, argon or mixtures thereof. Further, the feedcan contain water either as part of the carrier for the aldehydereactant feed or as a part of the nitrating agent.

The conditions and parameter ranges for conducting the homogeneousgaseous nitration of aldehydes are (a) that the reaction zone feed be ina molar ratio of NO₂ or equivalent to aldehyde of from about 0.3 to 4 orgreater and preferably from about 0.5 to 3. The environment can be,therefore, either a reducing or an oxidizing environment depending onthe feed ratio used. When oxygen is used as an additional feed, itshould be in from about 0.05 to 1 mole per mole of NO₂ or equivalent.The reaction is carried out at elevated temperature of from about 100°to about 500° C. and preferably from 150° to 400° C. The reaction iscarried out under elevated pressure of from about 5 to 20 bars with from5 to 12 bars being preferred. The combined temperature and pressureconditions must be such as to maintain the reactants in a homogeneousgas phase.

It has been found that by causing the homogeneous gaseous nitration tooccur under the combined elevated temperature and high pressureconditions stated herein one attains a high yield of nitrocompounds withvery high selectivity to a specific nitrocompound, RNO₂ as describedabove. These achievements are highly desired and attainable only underthe present reaction conditions.

The inert gases in the feed (A, CO, CO₂, N₂) can be from about 0 to 90volume percent. The water can be present in from about 0 to 30 weightpercent based on the NO₂ with at most 10 weight percent being preferred.The reaction contact time of the reaction gases in the reaction zone canbe from about 0.5 to 20 seconds with the order of from about 1 to 12seconds being preferred.

Referring to the drawing to illustrate the subject process, an aldehydesuch as acetaldehyde, etc. is transported from a reservoir (not shown)by pipeline 1 to preheater 2. Preheater 2 is also used to preheat thealdehyde being recirculated through pipeline 3, as more fully describedhereinbelow. The preheater is maintained at substantially the reactionzone entry temperature of about 100° to 500° C. and pressure of fromabout 5 to 20 bars. The preheated aldehyde is then passed throughpipeline 4 to reactor intake pipeline 5. The nitrogen dioxide and theoxygen (as air, when used) are introduced to preheater 8 via pipelines 6and 7, respectively. The preheater 8 is maintained at temperature andpressure conditions substantially the same as that of preheater 2. Themixed preheated NO₂ /O₂ gases pass through pipeline 9 to reactor intakepipeline 5 using gas-gas mixing devices such as spargers, venturis, etc.The preheated gases are passed through reactor 10 which may be in theform of a tubular reactor heated by salt at a temperature of from 100°to 500° C., preferably from 150° to 400° C. and at a pressure ofapproximately 5 to 20, preferably about 5 to 12 bars. The reactoreffluents withdrawn through pipeline 11 are cooled to ambienttemperature in cooler 12 which uses super-cooled water to rapidly coolthe gases. The cooled reactor effluents are separated in the separator13. The liquid effluent separates into organic liquid phase 14 andaqeuous liquid phase 15.

The uncondensed gaseous reaction effluents are removed from theseparator 13 through pipeline 16. The uncondensed gaseous reactioneffluents obtained in the present process are generally a mixture ofcomponents composed predominantly of nitrogen monoxide and inert gases.These effluent gases are distinctly different from those encountered inconventional hydrocarbon gaseous nitration processes where the effluentgases are rich in unreacted hydrocarbons. In such conventional processesthe unreacted hydrocarbons must be separated from the NO (which must beseparately treated) and recycled as part of the feed. Such separation iscomplex and costly. In contrast, the present uncondensed effluent gasesof separator 13 are substantially free of unreacted aldehyde and therebydo not require separation. Instead, these gases can be directly treatedat station 17 to re-oxidize the nitrogen oxide to nitrogen dioxide forreuse by, for example, directly injecting oxygen into the gaseouseffluent. To prevent build-up of inert gases due to the recycling ofgaseous effluent, a purge stream 18 is maintained.

The condensed organic and aqueous liquid phases 14 and 15, respectively,are removed from separator 13 and sent by pipelines 14' and 15' to anazeotropic distillation column 19. When the nitro compound product has alower density than water (i.e. some C₄ and higher nitro compounds) theorganic and aqueous liquid phases 14 and 15 will be in reversed positionin separator 13 to that shown. In such instances (not shown) line 14'will enter the bottom portion of column 19 and line 15' will enter thetop portion of column 19. Azeotropic distillation column 19 normallyoperates at a pressure of about 1.25 bars or less and at temperaturessufficient to azeotropically distill the nitroalkane or nitroaromaticproducts as well as other compounds having a boiling point lower thanthe nitro products, including any unreacted aldehyde feed withassociative water. These materials are passed via pipeline 20, condenser21 and pipeline 22 to distillation column 25. Some of the distillate maybe recycled to the azeotropic column 19 by pipeline 23. The majority ofthe water and the heavy by-products such as acids and the like areremoved as bottom products through pipeline 24. This stream containingheavy by-products may be recycled to reactor 10 via line 3.

The distillation column 25 operates at a pressure of about 1.25 bars orless and at a temperature range sufficient to remove overhead anyunreacted aldehyde as well as any other oxygenated hydrocarbonby-product, such as a temperature range of from 30° C. to 95° C. Thebottom product of column 25 is removed by pipeline 26 and is composed ofa mixture of a major amount of nitroalkanes or mixture of nitroalkanesor nitroaromatic as is appropriate based on the aldehyde feed used. Inaddition there may be present a small amount of water (from the priorazeotropic distillation) and trace amounts of oxygenated hydrocarbonby-products. The material removed by pipeline 26 is subsequentlychemically treated (not shown) to remove any trace contaminants then fedto a dehydration column (not shown) and where a mixture ofnitrocompounds are produced to a fractionation column (not shown) torecover pure nitro products. The nitro product of the present process iseither composed of a single nitro compound such as nitromethane or of amixture of nitro compounds highly selective with respect to one nitroproduct which is dependent on the starting aldehyde feed.

The overhead effluent of column 25 is removed by pipeline 27 throughcondenser 28. Some of the distillate may be recycled to column 25 bypipeline 29. The overhead distillate is made up predominantly ofaldehyde with small amounts of other oxygenated hydrocarbons such asacids, which can be readily recycled via pipeline 30 to preheater 2 aspart of the feed.

By utilizing an aldehyde as the feed material in the subject process ithas been unexpectedly found that one can readily form nitroalkane ornitroaromatic compounds, that the nitro compound product will be highlyselective based on the particular aldehyde used as the feed and that theunreacted aldehyde can be readily separated and recycled as part of thefeed to further improve the effectiveness and efficiency of the subjectprocess The subject process provides a means of custom directing theformation of a preselected nitro compound.

The following examples are given for illustrative purposes only and arenot meant to be a limitation on the claims appended hereto. All partsand percentages are by weight unless otherwise indicated.

EXAMPLE I

A series of production runs were conducted using acetaldehyde as thefeed. Each feed material was preheated to 150° C. at 10 bars. Thenitrogen dioxide and oxygen (when used) were preheated separately fromthe acetaldehyde and water (when used). The preheated feed materialswere then mixed and reacted in a tubular reactor for a residence time of8 seconds. The reactor effluent was quenched. The nitric oxide, carbonmonoxide and carbon dioxide were removed and the nitric oxide treatedwith oxygen to obtain nitrogen dioxide which was recycled to thereactor. The remaining liquid was distilled to azeotropically remove thenitro compound and low boiling oxygenated hydrocarbon includingunreacted acetaldehyde. The azeotropic distillate was further distilledto separate the nitro compound from the oxygenates. The nitro compoundwas nitromethane. No other nitro compound was detected.

EXAMPLE II

A series of production runs were conducted in the same manner as inExample I above except that the reaction conditions were varied. Detailsof the runs are given in Table I below:

                  TABLE I                                                         ______________________________________                                        Run No.          1       2       3     4                                      ______________________________________                                        Temperature (°C.)                                                                       150     190     190   250                                    Pressure (atm)   7.8     10.5    7.8   10.2                                   Feed (mmoles/hr)                                                              Acetaldehyde     2020    2263    2240  2265                                   Nitrogen Dioxide 1032    1198    1486  1311                                   Water            1925    1889    1886  1759                                   Oxygen           433     303     303   303                                    Nitrogen         4685    6587    6587  6587                                   NO.sub.2 /CH.sub.3 CHO                                                                         0.51    0.53    0.66  0.57                                   Acetaldehyde Conversion (%)                                                                    23.8    27.5    28.4  40.5                                   Nitromethane     21.0    25.5    24.6  23.0                                   Yield, %*                                                                     ______________________________________                                         *moles of nitromethane/total moles of C.sub.1 compounds produced         

While the invention has been described in connection with certainpreferred embodiments, it is not intended to limit the invention to theparticular form set forth, but, on the contrary, it is intended to coversuch alternatives, modifications and equivalents as defined by theappended claims.

What is claimed is:
 1. A process for selectively forming nitroalkanescomprising contacting in a reaction zone in a homogeneous gas phase andat an elevated pressure of from about 5 to about 20 bars, a temperatureof from about 100° C. to about 500° C. and a time of from about 0.5 to20 seconds at least one C₂ -C₁₀ aliphatic aldehyde with nitrogen dioxideand recovering the formed nitroalkane compound.
 2. The process of claim1 wherein the reaction zone further contains oxygen, water or both. 3.The process of claim 1 wherein the aldehyde is selected from a C₂ to C₅aliphatic aldehyde or mixtures thereof.
 4. The process of claim 2wherein the aldehyde is selected from a C₂ to C₅ aliphatic aldehyde ormixtures thereof.
 5. The process of claim 2 wherein the aldehyde isacetaldehyde.
 6. The process of claim 3 wherein the aldehyde isacetaldehyde.
 7. The process of claim 1 wherein the process furthercomprises cooling the reaction zone effluent, separating the resultingliquid phase effluent from the non-condensed gaseous effluent andrecovering any aldehyde therefrom and returning at least a portion ofsaid aldehyde to the reaction zone.
 8. The process of claim 2 whereinthe process further comprises cooling the reaction zone effluent,separating the resulting liquid phase effluent from the non-condensedgaseous effluent and recovering any aldehyde therefrom and returning atleast a portion of said aldehyde to the reaction zone.
 9. The process ofclaim 3 wherein the process further comprises cooling the reaction zoneeffluent, separating the resulting liquid phase effluent from thenon-condensed gaseous effluent and recovering any aldehyde therefrom andreturning at least a portion of said aldehyde to the reaction zone. 10.The process of claim 4 wherein the process further comprises cooling thereaction zone effluent, separating the resulting liquid phase effluentfrom the non-condensed gaseous effluent and recovering any aldehydetherefrom and returning at least a portion of said aldehyde to thereaction zone.
 11. The process of claim 5 wherein the process furthercomprises cooling the reaction zone effluent, separating the resultingliquid phase effluent from the non-condensed gaseous effluent andrecovering any aldehyde therefrom and returning at least a portion ofsaid aldehyde to the reaction zone.
 12. The process of claim 8 whereinthe process further comprises cooling the reaction zone effluent,separating the resulting liquid phase effluent from the non-condensedgaseous effluent and recovering any aldehyde therefrom and returning atleast a portion of said aldehyde to the reaction zone.
 13. The processof claim 2 wherein the reaction zone pressure is from about 5 to 12bars, temperature is from about 150°-400° C., the O₂ to NO₂ molar ratiois from about 0.05 to 1 and the NO₂ to aldehyde molar ratio is fromabout 0.3 to
 3. 14. The process of claim 4 wherein the reaction zonepressure is from about 5 to 12 bars, temperature is from about 150°-400°C., the O₂ to NO₂ molar ratio is from about 0.05 to 1 and the NO₂ toaldehyde molar ratio is from about 0.3 to
 3. 15. the process of claim 7wherein the reaction zone pressure is from about 5 to 12 bars,temperature is from about 150°-400° C., the O₂ to NO₂ molar ratio isfrom about 0.05 to 1 and the NO₂ to aldehyde molar ratio is from about0.3 to
 3. 16. The process of claim 8 wherein the reaction zone pressureis from about 5 to 12 bars, temperature is from about 150°-400° C., theO₂ to NO₂ molar ratio is from about 0.05 to 1 and the NO₂ to aldehydemolar ratio is from about 0.3 to
 3. 17. The process of claim 10 whereinthe reaction zone pressure is from about 5 to 12 bars, temperature isfrom about 150°-400° C., the O₂ to NO₂ molar ratio is from about 0.05 to1 and the NO₂ to aldehyde molar ratio is from about 0.3 to
 3. 18. Theprocess of claim 11 wherein the reaction zone pressure is from about 5to 12 bars, temperature is from about 150°-400° C., the O₂ to NO₂ molarratio is from about 0.05 to 1 and the NO₂ to aldehyde molar ratio isfrom about 0.3 to 3.